Yes? - I've been there and done that and got it entirely wrong. Last Christmas Eve, I came by an unbuilt QCX kit from about 2017. I built it up on Christmas Eve evening and had it working by midnight. Then, listening on my Kiwi SDR , and transmitting on a dummy load, I could see sprogies all over the place at harmonic intervals and elsewhere too. I was simply overwhelming the receiver and creating mirages.? I'm sure that if you built the LPF to the specs provided and have been able to tune it for good power output, you have done it right and your transmitter will be fine.
If you really do think something is amiss, get someone to check the harmonics? a couple of miles away when you are transmitting on an aerial. Or you could use one of those old fashioned wave meter things we used to build in the 1980s. They were a sort of crystal set with a variable capacitor, a suitable coil and a small meter. You could scan for the fundamental, adjust the meter sensitivity for full scale and then slowly open the vanes of the variable cap and look for peaks at the harmonics.?
